God purposefully designed us to pursue happiness.
Does seeking your own happiness sound self-centered? Aren't Christians supposed to seek God, not their own pleasure? To answer this question we need to understand a crucial truth about pleasure-seeking (hedonism): we value most what we delight in most. Pleasure is not God's competitor, idols are. Pleasure is simply a gauge that measures how valuable someone or something is to us. Pleasure is the measure of our treasure.

We know this intuitively. If a friend says to you, "I really enjoy being with you," you wouldn't accuse him of being self-centered. Why? Because your friend's delight in you is the evidence that you have great value in his heart. In fact, you'd be dishonored if he didn't experience any pleasure in your friendship. The same is true of God. If God is the source of our greatest delight then God is our most precious treasure; which makes us radically God-centered and not self-centered. And if we treasure God most, we glorify Him most.

Christian Hedonism is not a contradiction after all. It is desiring the vast, ocean-deep pleasures of God more than the mud-puddle pleasures of wealth, power or lust. We're Christian Hedonists because we believe Psalm 16:11, “You will show me the way of life, granting me the joy of your presence and the pleasures of living with you forever" NLT

Selfishness is a matter of what you value and where your focus is. Pursuing God because we delight in Him, however, puts the focus on God and not ourselves. And since God is what we delight in most, it means we value Him most. This makes pursuing God out of joy God-centered, not self-centered (and therefore selfish) since our focus and value is God, not ourselves.